While it is widely recognized that reduction of blood flow in arties is a major factor in heart disease, there are many medical conditions in which restricting or occluding blood flow through a blood vessel is desirable. For example, sclerotherapy is the injection of a sclerosing agent (such as morrhuate sodium) into a blood vessel to produce inflammation and scarring; injection of the sclerosing agent closes the lumen, and is followed by shrinkage and hardening of the vessel. Sclerotherapy is often used to treat varicose veins.
Yet another medical condition for which occluding blood flow is an effective therapy is cirrhosis, which can lead to the development of esophageal and gastric varices. When portal venous flow becomes obstructed (for example, in cirrhosis of the liver, distorted hepatic sinusoids restrict the flow of blood through the liver), varices can develop, creating a collateral circuit that ‘bypasses’ the portal venous system and returns blood to the systemic circulation. A high rate of bleeding is associated with such varices, often leading to increased morbidity. Improving survival in patients with cirrhotic livers may provide such patients with additional time necessary to obtain a more definitive therapy, such as a liver transplant. Current methods for preventing bleeding from such varices include the use of non-selective beta-blockers and/or nitrates, endoscopic therapy (band ligation or injection therapy), transjugular intrahepatic portosystemic shunts, or surgical decompression of the portal venous system. Injection therapy is based on introducing a sclerosant agent into the vessel. Such agents cause local irritation to the vessel, resulting in rapid intravascular thrombus formation. Although injection therapy is effective in occluding vessels and arresting active bleeding, ulceration of the mucosa and bacteremia are common side effects, and there is potential for serious complications, such as perforation, acute respiratory distress syndrome (ARDS), and thromboembolic events.
In the United States, the most common cause of cirrhosis is a hepatitis C infection. It is estimated that well over 4 million Americans have been infected with the hepatitis C virus. Since the hepatitis C infection is a chronic infection that progresses over decades, the prevalence of cirrhosis due to hepatitis C infection is expected to increase in years to come. Some projections estimate that the number of cases of cirrhosis due to the hepatitis C will increase from about 450,000 in 2000 to almost 900,000 by the year 2020. Studies indicate that 60-90% of patients who develop cirrhosis go on to develop esophageal and/or gastric varices. Of patients who develop varices, 30-40% have bleeding that is attributed to varices. Of those patients who experience variceal bleeding, 20-35% will die as a result of their initial variceal bleed, and, of those who survive, 70% will experience a recurrent episode of variceal bleeding within one year. Clearly, variceal bleeding is a major cause of morbidity and mortality in patients with cirrhosis, and it would be desirable to provide new therapies that can be used to selectively occlude blood flow in a blood vessel, and thus, to treat variceal bleeding and other medical conditions (such as varicose veins).
Ultrasound has gained acceptance as an imaging technique particularly well suited to providing information about a patient's internal structures without risk of exposure to potentially harmful radiation, as may occur when using X-ray imaging techniques. While often used as an imaging tool, at higher intensities, ultrasound can induce biological effects (bioeffects), including thermal effects and mechanical effects. The thermal effects are generally the result of the absorption of acoustic energy, and the mechanical effects are generally based on the cavitation produced by gas-filled bubbles. Several different medical therapies that are based on the bioeffects of ultrasound have been studied. Because ultrasound can pass through tissue, ultrasound is generally non-invasive, and non-invasive/minimally invasive therapies are growing in popularity. Advantages of such non-invasive/minimally invasive therapies include reduced blood loss, reduced risk of infection, shorter hospital stays, and generally lower health care costs.
One ultrasound therapy that has been investigated is the use of high intensity focused ultrasound (HIFU) to destroy abnormal tissue and to stop bleeding. While HIFU has been shown to be effective in arresting bleeding and occluding vessels, significant thermal injury to perivascular tissue often results from such therapy. While there are certainly medical conditions where such tissue damage is acceptable (for example, to stop bleeding in trauma victims, since the failure to stem blood loss will result in death), such damage is likely to be unacceptable for the treatment of varicose veins and varices as discussed above. It would therefore be desirable to provide an ultrasound based minimally invasive therapy for occluding blood flow in vessels with minimal damage to adjacent tissue. Such a therapy can be employed for hemostasis and sclerotherapy.